Experimental study on restraining outflow distortion of a waterjet inlet using vortex generator jets

The internal flow of the flush-type waterjet intake duct is simulated through an experiment in a low speed wind tunnel. The distributions of static pressure along the intake duct wall and the total pressure in the pump face plane are measured to investigate the influence of the vortex generator jet (VGJ) on the outflow behavior. Test cases were screened by Taguchi method. Experiment results verify a pair of VG jets with a blowing ratio 2 and lateral spacing 4 has the best performance. The inlet’s outflow is improved that total pressure recovery coefficient is raised from 0.712 to 0.771 and uniformity coefficient of axial velocity component from 0.652 to 0.824. The distortion coefficient of total pressure is reduced from 0.381 to 0.177. Vortex generator jet is a promising active flow control for waterjet application.


Introduction
Water jet propulsion is a new method of marine propulsion which is applicable to high-speed and shallow water navigating ships and amphibious vehicles.The intake pipe provides the inlet flow to the water jet pump on a stern deck.Due to the arrangement limitation of a hull structure, the inlet flow turns twice in the duct and forms vortices in the intake pipe.The distorted inlet flow before the pump affects greatly the hydrodynamic characteristics and hydraulic loads of the propulsion pump.Wang et al [1] concluded in the reduced pump efficiency due to the distorted pump inlet flow.
Zhang et al [2] extended the inlet pipe and reduced the ramp slope of the inlet pipe to improve the duct performance.Huang et al [3] reduced the flow distortion in the inlet pipe and improved the total flow pressure recovery coefficient by a parametric multi-objective optimization algorithm, but the thruster efficiency was not raised significantly.
Vortex generator jet (VGJ) is an active control method developed after the vortex generator.It can be switched off when it is not necessary and can be adapted to a wide range of operating conditions.There is a wealth of research on the application of the VGJ in aircraft intakes.Johnston et al [4,5] used the VGJ to eliminate turbulence separation.Ball [6] used side-blowing with 2% of inlet flow to achieve effective boundary layer control and eliminate separation in S-shaped intakes.Ng et al [7] demonstrated that VGJ jets can achieve the same separation suppression effect and reduce total pressure loss as VG in a square S-bend.Zhou et al [8] arranged VGJ on a water jet propulsion inlet pipe and optimized the jet parameters by numerical simulation to derive a VGJ scheme that can effectively suppress the slope side flow separation and improve the flow quality of the runner outflow.
In this paper, we designed an inlet pipe flow on a low-speed wind tunnel to simulate the influence of VGJ installation and investigated its best parameters on the performance of the inlet pipe.Our work intends to fill the gap in an experimental study of using VGJ to control the flow of the inlet pipe, and to provide an experimental datum for the engineering application of VGJ to improve the efficiency of a waterjet propulsor.

Intake pipe model
A typical intake pipe with an inlet slope of 25°(similar to Brandner et al. [9]) was chosen for the experimental model, as shown in figure 1.The diameter of the outlet pipe is D=100 mm.The total length from the inlet to the outlet of the bend section is 4.6 D, and the vertical distance between the outlet centerline and the plane of the inlet is 1.25D.The entrance is composed of a rectangle and half an ellipse.The duct cross section deforms gradually from the entrance to the round pipe exit.
On the flow channel ramp side wall, fifteen static pressure taps are arranged along the centerline uniformly.In the outlet plane, eight pressure taps are set on the peripheral line.Each tap is numbered as shown in figure 1.

Measurement methods
The experiments were conducted on a low-speed open wind tunnel, and the experimental system is illustrated in figure 2. Due to the large size of the actual ship water jet thruster, even the scaled-down experiment requires a huge water tank, which is an expensive apparatus and difficult for measuring flow field.Jung et al [10] and Wu et al [11] used a low-speed wind tunnel instead of the water tank experiment to study the inlet flow performance, which provided an experimental basis for the design of a highefficiency water jet thruster.The experimental wind speed is 10m/s, the simulated air speed is V0, and the average turbulence of the airflow at the exit of the wind tunnel is 1.5%.The inlet pipe model is mounted on a rectangular pipe with the cross-section size of the pipe 400mm×600mm simulating the control body of flow field under a ship bottom hull.A suction fan is installed at the inlet exit.A flow nozzle is used to regulate the inlet velocity ratio (IVR, the ratio of the outlet mean flow velocity Vi to the freestream V0) by controlling the fan speed.On the mid-axis of the inlet surface, the boundary layer displacement thickness was measured δ = 22mm by a five-hole pitot tube.The experimental data were compared with the velocity distribution at the bottom of the ship derived from Wieghardt's formula [14] as shown in figure 3. The velocity profile has a shape exponent of n=6, which locates between the laminar flow (n=4) and a fully turbulent (n=9).A Y-shaped total pressure rake with 3 spokes was installed in the PF cross-section to measure the total pressure, each spoke had 5 measuring probes with a diameter of 1 mm, distributed according to an equal ring area, together with the total pressure probe at the center, 16 probes were distributed from the outside to the inside in on five rings, as shown in figure 4, and the blocking ratio by the rake was 1.45%.The total pressure rake was rotated circumferentially in 7 positions to obtain 121 measurement points to cover the measurement surface for a section contour.The sampling frequency of the pressure measurement was 1 Hz with an accuracy of ±0.5% and a sampling period of 30 seconds.
Each pitot tube is fixed direction on the rake, which does not take into account the swirling angle in the cross-section.Referring to the measured flow angles at the outlet of a large S-inlet by Xu [12] with a five-hole probe, and the numerical simulation by Huang et al [13], the swirling angle is less than 10°.Thus the uncertainty of the total pressure measurement is inferred equivalent to sin2T=3%, and the obtained distributions is reliable to reflect the total pressure distortion in the PF plane.

Vortex generator jet
The initial VGJ arrangement is shown in figure 5. Two jet holes are placed at 1.0D in front of the inlet entrance, symmetrically distributed about the inlet centerline.The diameter of the jet hole is d=5mm.A long pipe of 10D length serves the jet fluids and provides steady jet flow.Fluid is ejected at a fixed pitch angle β of 45°.Manipulated parameters are jet skew angle α, hole spacing X/δ and jet blowing VR (Ve/V0).

Design of experiment
Flow control effects of VGJ device depend on jet parameters.Three factors of blowing ratio VR, skew angle α and hole spacing X/δ were specified with given ranges such as VR=1~3, α=30°~90°, and X/δ=2~6 [5].Experimental tests are tabulated by Taguchi L9(3 3 ) sampling in figure 6 The experiment was repeated three times at the level of each factor, and the levels of every two factors comprised a comprehensive experimental program.These two features allowed the experimental points covering evenly the experimental range.In order to measure inlet channel performance, we have defined three important parameters, which are total pressure recovery coefficient(φ) uniformity coefficient(ξ) and distortion coefficient of total pressure(DC60).DC60 is an important index for evaluating the compatibility of intake channels and propulsion pumps.High DC60, low ξ and φ inlet channel outflow will affect the normal operation of the water jet propulsion pump, which will not only cause the efficiency of the propulsion pump to be reduced, but also cause it to generate abnormal vibration and noise.Table 1 shows the variation of the three parameters for the nine cases.It is obvious to see from the parameters of case 5,6,8 and 9 that it has a better optimization performance compared to several others.These cases were therefore selected for further study.

VGJ effect at design condition
Figure 7 compares the total pressure distribution at the PF surface for the VGJ with the 4 cases and the prototype at the design condition of IVR=0.7.In the uppermost contour, the high-pressure region is distributed in a rectangular shape in the lower half of the PF, while the low-pressure region appears in different degrees at the top and bottom of the outflow surface, with the low-pressure region in the top half.Firstly, from case5, 6(VR=2) and case8, 9(VR=3), It is apparent to show that as the VR increases, the level of pressure at the bottom of the PF is also increased.This is due to the fact that VR affects the vortex volume and vortex core location of the flow vortex, and the higher the jet velocity, the larger the influence range of the flow vortex, the farther it is from the wall, and the location is closer to the bottom of the flow channel when it enters the inlet channel, so it can inhibit the lip side flow separation to a certain extent and increase the bottom pressure of the PF.As can be seen from Case 6,9 and 5 (corresponding to X/δ=2~6 respectively), the pair of streamwise vortices generated by the VGJ with X/δ = 2 are closely spaced and have a greater ability to lift the mainstream energy toward the top of the flow channel, but at the same time the high-pressure region is squeezed, which leads to a certain energy deficit.However, when X/δ=6, the distance between the jet holes is slightly larger than the inlet width of the inlet channel, at this time, the VGJ has no obvious effect, and is almost the same as the original case.Therefore, the hole spacing X/δ = 4 works better.

Original
No.5 No.6 No.8 No.9 Figure 8 gives the distributions of the static pressure coefficient along the ramp centerline and the peripheral PF plane.From figure 8(a), it can be seen that the static pressure increases gradually along the inlet ramp wall.The pressure distributions are similar before the 7 th tap, but diverse from tapping 9.
Thereafter, there is a clear pressure "plateau" in the prototype inlet pipe near the measurement hole 8, which indicates that the flow loses the sustained deceleration and diffusion and separates here.These 4 cases with different VGJ all can enhances the pressure after the separation point, indicating that the VGJ inhibits the flow separation and maintains the continuous flow diffusion.In these Case No.6and 9 that have same α=90°, high VR and small hole spacing, have the best inhibition of flow separation.It can also be related to similar profiles in the early work of Johnston [4] where employs spanwise arrays of small, skewed, and pitched jets from holes in the surface.We can find that it has similar movement of streamwise distributions of the wall static pressure, although it is on the different test surface.Figure 8(b) reflects the annular pressure distribution in the PF plane.The pipe wall pressure decreases until the tapping 5 which is at the pipe bottom.It resembles the pressure distribution found in the paper [11].The VGJ elevates the overall PF static pressure level, including the bottom of the flow channel and the slope side pressure.Hydrodynamic performance is compared in table 2 to show the flow control effect of the VGJ with these cases.It confirms that intake duct with the best performance is the case No.6 and 9.In the case No.6, the total pressure recovery coefficient increased by 0.045 from 0.712 to 0.757, the uniformity coefficient increased by 0.162 from 0.652 to 0.814, and the total pressure distortion coefficient decreased by 0.333 from 0.640 to 0.307.In the case No.9, the coefficient of Φ and ξ increased by 0.026 and 0.119 respectively, and DC60 decreased by 0.351.It can then be concluded that the preferred VGJ combination of α=90°, high VR and small hole spacing works well and achieves its design goals.Therefore, further testing is carried to verify its effectiveness.We conducted several sets of experiments to verify the effect of VR in the inlet channel under the VGJ combination of VR=2, α=90°, and X/δ=4.Figure 10(a) shows that the three evaluation coefficient indexes change as VR increases, but the change in VR from 2 to 3 is almost negligible.On the other hand, figure 10(b) graphs the sensitivities of three evaluation indices with respect to jet parameters.It can be seen that pressure recovery is the least sensitive, but uniformity and distortion are sensitive to jet parameters.Balancing the evaluation among three factors, the best jet parameters are selected as a portfolio of VR=2, α=90°, and X/δ=4, which is consistent with the results of the previous argument.

VGJ effect at off-design conditions
The distortion degree in the PF plane is related to the pump working condition marked by IVR values.Thus, it is necessary to understand the VGJ effects at its off-design conditions.Figure 12 shows the comparison of the total pressure distributions at the designed IVR=0.7 and other three IVR conditions.It is clear that VGJ effect is maintained at IVR=0.3 and 0.7, but its effect is also evident for IVR=1.0.A common feature is that VGJ enlarges the core region of high pressure.At the beginning of ramp flow separation, i.e.IVR=0.5, jet brings additional momentum into the separation zone and depresses the total pressure deficit in the pipe top.For larger IVR>0.7,there is no flow separation which the jet is aimed at.But for less IVR<0.5, jet is not enough strong to dismiss all separation zone.Little adverse effect is found by adding VGJ except the extra power driving the VG jets.

Conclusions
In this paper, a flush waterjet inlet is investigated to use vortex generator jets to improve its performance.Pipe wall static pressure and outflow cross-section total pressure distributions are measured based on a wind tunnel experiment.Optimal VGJ parameters are explored through Taguchi experiment design.Conclusions are drawn as: (1) The jet velocity ratio VR, skew angle α and hole spacing X/δ of the VGJ have a primary and secondary bias on the inlet pipe outflow quality.The most influential factor on Φ is VR, the most influential factor on lifting ξ is α, and the most significant factor on suppressing DC60 is X/δ.
(2) Considering the effects of each factor on the performance index of the inlet pipe, the VGJ with jet velocity ratio VR=2, yaw angle α=90°, and hole spacing X/δ=4 has the best flow control.At IVR=0.7, the VGJ at this level combination increases Φ from 0.712 to 0.771, ξ from 0.652 to 0.824, and DC60 decreases from 0.381 to 0.177.
(3) VGJ restrains the flow distortion due to ramp flow separation at low IVR conditions.It has small effect on the inlet flow efficiency and pressure recovery.Final benefit is obtained by the potential improvement of pump inlet working conditions and thus the whole device performance.

Figure 1 .
Figure 1.Inlet model and pressure tapping arrangement

Figure 3 .
Figure 3. Velocity profile in the boundary layer before inlet entrance.

Figure 8 .
Figure 8. Inlet wall pressure distributions.The effect of outflow non-uniformity (distortion) on the pump inlet flow is investigated at the outlet face of the inlet pipe (also the pump inlet flow face PF).The velocity distribution along the PF plumb line is shown in figure9.The local velocity is normalized by the averaged flow velocity Vi.The PF velocity distribution is extremely nonuniform.Variation amplitude is up to 1.5 times Vi.The flow velocity profile has a peak value at 30% radius close to the pipe bottom and decreases rapidly towards the top of the PF, leading to a poor outflow uniformity in the core flow region.It is clear that Case No. 6 effectively increased the flow velocity in the upper middle of the PF, generating a velocity plateau that substantially improved the PF incoming flow uniformity.Although the other cases also improved the velocity distribution, the results were not as good as in Case No. 6.

Figure 10 .
Figure 10.Performance sensitivities with respect to jet parameters.Figure11(a) shows the velocity distribution of the flow field at 0.5D in front of the influent plane under the best VGJ combination.As we can see in the figure 11(a) that VGJ generates a pair of symmetric flow vortices with opposite rotational directions and comparable strengths, and the vortex nuclei are offset with respect to the jet orifice in both the normal and spreading directions.Figure 11(b) shows the velocity distribution at the 2D cross section behind the influent plane.We know that slow fluid flow on the slope side of the inlet runner creates lateral flow velocities, which in turn creates a secondary flow to form a reflux zone, leading to flow separation and affecting the performance of the inlet runner and the quality of the outflow.It can be seen that after the installation of VGJ, the flow vortex continues to develop along the flow direction close to the side wall surface of the slope.The VGJ generates two symmetric flow vortices squeezing the high velocity zone in the middle, facilitating the exchange of mainstream energy and slope-side boundary layer energy.
Figure11(a) shows the velocity distribution of the flow field at 0.5D in front of the influent plane under the best VGJ combination.As we can see in the figure11(a) that VGJ generates a pair of symmetric flow vortices with opposite rotational directions and comparable strengths, and the vortex nuclei are offset with respect to the jet orifice in both the normal and spreading directions.Figure11(b)shows the velocity distribution at the 2D cross section behind the influent plane.We know that slow fluid flow on the slope side of the inlet runner creates lateral flow velocities, which in turn creates a secondary flow to form a reflux zone, leading to flow separation and affecting the performance of the inlet runner and the quality of the outflow.It can be seen that after the installation of VGJ, the flow vortex continues to develop along the flow direction close to the side wall surface of the slope.The VGJ generates two symmetric flow vortices squeezing the high velocity zone in the middle, facilitating the exchange of mainstream energy and slope-side boundary layer energy.

Figure 11 .
Figure 11.Velocity distribution for different cross-section.

Figure 12 .Figure 13 Figure 13 .
Figure 12.Off design VGJ effect on total pressure distribution in PF plane.

Table 2 .
Hydrodynamic inlet performance with the design VGJ.