Vortex Generator Jets Research Articles

Longitudinal Vortex Structure in the Separating Shear Layer over the Backward Facing Step with Vortex Generator Jets (The Effect of Injection Pitch)

The paper presents the flow mechanism of a reattachment process over a backward facing step with vortex generator jets (VGJs). The spanwise arrays of VGJs upstream of the step introduces the corotating longitudinal vortices in a separating shear layer. The measurement of the longitudinal vortices by the hot wire anemometer is made to clarify the spanwise structure in the separating shear layer downstream of the step. The particular emphasis is placed on understanding of the effect of the jet injection pitch on the shear layer development. The spanwise structure in the case of the injection pitch of 20 times jet diameter shows two-dimensional feature at the bottom of the separating shear layer as in the distribution of forward flow fraction near the reattachment region.

Longitudinal Vortex Structure of the Backward Facing Step Flow with Vortex Generator Jets (Turbulent Flow Characteristics)

The paper presents the turbulent flow characteristics of a separating shear layer associated with the corotating longitudinal vortices introduced by the vortex generator jets (VGJs) over a backward facing step. The turbulent flow structure is measured by the hot-wire anemometer. The two-dimensional time averaged velocity profile in the separating shear layer downstream of the step is observed, while the turbulent kinetic energy and Reynolds stress have the spanwise structure depending on the VGJ injection location. VGJ produces the primary and secondary vortices. The behavior of the primary and secondary vortices in the separating shear layer is also discussed.

4037 縦渦発生ジェットによる流れ場変化に適合したはく離の能動制御(G05-8 境界層・はく離,G05 流体工学)

Flow separation is mostly an undesirable phenomenon and boundary layer control is an important technique for flow separation problems on airfoils and in diffusers. The vortex generator jet method is one of the boundary layer control technique and an active control technique which provides a time-varying control action to optimize performance under a wide range of flow conditions because the strength of longitudinal vortices can be adjusted by varying the jet speed. In this study, an adaptive separation control system using vortex generator jets with rectangular orifices has been developed. The separation control system can be practically applied to the flow separation control of a two-dimensional diffuser. It was confirmed that our separation control system could adaptively suppress flow separation for the flow fields caused by some changes in freestream velocity and the divergence angle of the diffuser. Furthermore, we developed the system which starts the jet blowing just before the onset of separation.

10304 はく離の前兆となる流れ場変動(流体(1))

Jets issuing through small holes in a wall into a freestream have proven effective in the control of flow separation. This technique is known as the vortex generator jet method (VGJ). If a precursor signal of separation is found, the separation control system using VGJ can be operated just before the onset of separation. Therefore, flow field with no separation is always attained. In this study, we measured the flow field and the wall static pressure in a two-dimensional diffuser. Flow separation occurs when the divergence angle reaches at 18 degrees. The streamwise velocity measurements were carried out in the separated shear layer and the spectral analysis was applied to the velocity fluctuations at some angle of diffuser. The pattern of peak in the spectral analysis is changed as the divergence angle increases over 18 degrees due to the enhancement of the growth of the shear layer vortices.

20308 はく離制御効果に与える縦渦発生ジェット吹出し口形状の影響(流れの計測と制御(2),OS6 流れの計測と制御)

20308 はく離制御効果に与える縦渦発生ジェット吹出し口形状の影響(流れの計測と制御(2),OS6 流れの計測と制御)

10310 非円形吹出し口を用いた縦渦発生ジェット吹出し口近傍における渦構造の可視化(流体(2))

10310 非円形吹出し口を用いた縦渦発生ジェット吹出し口近傍における渦構造の可視化(流体(2))

Effect of Direction of Triangular Orifice for Vortex Generator Jets on Separation Control

Jets issuing through small holes into a freestream have proven effective in the control of boundary layer separation. The beneficial effect of separation control is obtained only if the jets are pitched to the lower wall and skewed with respect to the freestream direction. In the previous study, the suppression effect of the orifice shape on flow separation was reported for three types of jet orifice (circular, triangular and square orifices). The triangular orifice generates the strong vortex and makes effective the pressure recovery in the diffuser in comparison with the circular and square orifices. In this study, four types of the jet orifice (model-U, model-D, model-R and model-L) are installed and the effect of the direction of the vertex of the triangular orifice on separation control is investigated in a two dimensional diffuser. Separation control is accomplished by all models, however the suppression of flow separation is most ineffective among the four models in the case of model-L (the vertex points to the direction of the jet pitch angle).

Numerical Investigation of Low-Pressure Turbine Blade Separation Control

Laminar separation on the suction side of low-pressure turbine (LPT) blades at low operating Reynolds numbers can degrade overall engine efficiency and impose limitations on the flight envelope. In wind-tunnel experiments it was shown that laminar separation can be controlled by pulsed vortex generator jets. This active-flow-control technology could be transferred to real flight hardware with more confidence if the physical mechanisms involved in the control were better understood. Here, calculations of a linear LPT cascade at a Reynolds number based on axial chord of 2.5 x 10 4 are presented

Numerical Simulation of Separation Control for Transitional Highly Loaded Low-Pressure Turbines

The subsonic flow through highly loaded low-pressure turbines is simulated numerically using a high-order method. The configuration approximates cascade experiments that were conducted to investigate a reduction in turbine stage blade count, which can decrease both weight and mechanical complexity. At a nominal Reynolds number of 25 × 10 3 based upon axial chord and inlet conditions, massive separation occurs on the suction surface of each blade as a result of uncovered turning. Pulsed injection vortex generator jets were then used to help mitigate separation, thereby reducing wake losses. Computations were performed for both uncontrolled and controlled cases and reproduced the transitional flow occurring in the aft-blade and near-wake regions. The numerical method utilizes a centered compact finite difference scheme to represent spatial derivatives, which is used in conjunction with a low-pass Pade-type nondispersive filter operator to maintain stability. An implicit approximately factored time-marching algorithm is employed, and Newton-like subiterations are applied to achieve second-order temporal accuracy. Calculations were carried out on a massively parallel computing platform, using domain decomposition to distribute subzones on individual processors. A high-order overset grid approach preserved spatial accuracy in locally refined embedded regions. Features of the flowfields are elucidated, and simulations are compared with each other and with available experimental data. Relative to the uncontrolled case, it was found that pulsed injection maintained attached flow over an additional 15% of the blade chord, resulting in a 22% decrease of the wake total pressure loss coefficient.

Effect of Vortex Generator Jets with Different Aspect Ratios on Boundary Layer Control

The vortex generator jets can achieve the adaptive control by adjusting properly the jet speed in response to flow situations. The vortex generator jet method may accomplish separation control only when it is necessary, and therefore it is useful for both design and off-design conditions. In this study, the effect of vortex generator jets on separation control is investigated for three types of rectangular orifices with aspect ratios AR=0.16, 1 (square) and 6.4. The rectangular orifice with AR=0.16 set the long side of the orifice in the streamwise direction and the long side turned to the spanwise direction for AR=6.4. For the rectangular orifices with AR=6.4, the vortices become stronger and exist at lower vertical positions in comparison with those for the other orifice types. Therefore, the rectangular orifices with AR=6.4 promote the boundary layer mixing and make effective the separation control.

234 はく離直前の流れ場変動(G05-3 剥離制御・抵抗低減,G05 流体工学)

Jets issuing through small holes in a wall into a freestream have proven effective in the control of flow separation. This technique is known as the vortex generator jet method (VGJ). If a precursor signal of separation is found, the separation control system using VGJ can be operated just before the onset of separation. Therefore, flow field with no separation is always attained. In this study, to generate managed separation, the divergence angle of the diffuser was controlled by a stepping motor. Flow separation occurs when the divergence angle reaches at 20 degrees in our experimental condition. The distributions of the velocity and turbulence intensity of the air flow were measured by using an X-type hot wire probe. The vertical turbulent intensity at the upstream location of the divergent portion is enhanced before the onset of separation.

Vortical Structure in the Near Field of Vortex Generator Jets

Vortical Structure in the Near Field of Vortex Generator Jets

Secondary Flow Control Using Vortex Generator Jets

In this paper, results are presented of an experimental investigation into the effectiveness of vortex generator jets in controlling secondary flows in two-dimensional S-duct diffusers. The experiments were performed in uniform and distorted inflow conditions and the performance evaluation of the diffuser was carried out in terms of static pressure recovery and quality of the exit flow. In the case with inflow distortion, tapered fin vortex generators were employed in addition to vortex generator jets to control flow separation that was detected on the wall with inflow distortion. Detailed measurements including total pressure, velocity distribution, surface static pressure, skin friction, and boundary layer measurements were taken at a Reynolds number of 7.8×105. These results are presented in terms of static pressure rise, distortion coefficient, and total pressure loss coefficient at the duct exit. For uniform inflow, the use of vortex generator jets resulted in more than a 30 percent decrease in total pressure loss and flow distortion coefficients. In combination with passive device (tapered fin vortex generators), the vortex generator jets reduce total pressure losses by about 25 percent for distorted inflow conditions. A potential application of this method may include control of secondary flows in turbo machinery.

A Separation Control of the Diffuser Flow by the Vortex Generator Jets

In order to establish the active control system by using the vortex generator jets (VGJs), it is necessary to understand the detailed suppression mechanism of the separated flow in the diffuser. An opening angle of the diffuser can be changed from 8 to 15 deg corresponding to three flow regimes, “Unstalled”, “Appreciable Stall” and “Transitory Stall”. A micro wall flow sensor was used for measurement of forward flow fraction. It is found that VGJ makes the local forward flow fraction increase and prevents the flow detachment. Although the effect of separation control is obtained resulting from the higher jet velocity ratio, excessive jet velocity ratio introduces lower diffuser effectiveness due to the mixing loss.

後方ステップ流れにおける渦発生ジェットによる縦渦の挙動(S22-6 境界層,管内流の制御-2,S22 流れ制御の技術)

後方ステップ流れにおける渦発生ジェットによる縦渦の挙動(S22-6 境界層,管内流の制御-2,S22 流れ制御の技術)

Effect of Jet Orifice Shape of Vortex Generator Jets on Separation Control.

Effect of Jet Orifice Shape of Vortex Generator Jets on Separation Control.

Effects of Pulsing on a Vortex Generator Jet

The evolution of a pulsed vortex generator jet embedded in a turbulent boundary layer was examined experimentally. The jet, which was pitched 45 deg and skewed 90 deg, had a velocity three times greater than the freestream. The velocity e eld in planes normal to the freestream was measured by the particle-image-velocimetry method at four stations downstream of the jet exit. The pulsed jet created a starting vortex ring followed by a pair of counter-rotating streamwise vortices, one of them being markedly stronger. Phase-averaged data indicate that the maximum circulation and peak vorticity of the stronger vortex are approximately 30% greater than the average values for a steady jet with the same velocity. However, circulation averaged over the entire pulse was less than that fora steady jet at the samelocation. Thecoreof theprimary streamwisevortex penetratesapproximately 50% farther into the boundary layer than a steady jet with the same velocity. The larger penetration takes place during the initial portion of the pulse and is caused by the jet starting vortex ring.

Development of a Steady Vortex Generator Jet in a Turbulent Boundary Layer

The development of a vortex generator jet within a turbulent boundary layer was studied by the particle image velocimetry method. Jet velocities ranging from one to three times greater than the freestream velocity were examined. The jet was pitched 45 deg and skewed 90 deg with respect to the surface and flow direction, respectively. The velocity field in planes normal to the freestream was measured at four stations downstream of the jet exit. The jet created a pair of streamwise vortices, one of which was stronger and dominated the flow field. The circulation, peak vorticity, and wall-normal position of the primary vortex increased linearly with the jet velocity. The circulation and peak vorticity decreased exponentially with the distance from the jet source for the jet-to-freestream velocity ratios of 2 and 3. The wandering of the streamwise vortex can be as much as ±30% of the local boundary layer thickness at the farthest measurement station.

Separation Control on Low-Pressure Turbine Airfoils Using Synthetic Vortex Generator Jets

Oscillating vortex generator jets have been used to control boundary layer separation from the suction side of a low-pressure turbine airfoil. A low Reynolds number (Re=25,000) case with low free-stream turbulence has been investigated with detailed measurements including profiles of mean and fluctuating velocity and turbulent shear stress. Ensemble averaged profiles are computed for times within the jet pulsing cycle, and integral parameters and local skin friction coefficients are computed from these profiles. The jets are injected into the mainflow at a compound angle through a spanwise row of holes in the suction surface. Preliminary tests showed that the jets were effective over a wide range of frequencies and amplitudes. Detailed tests were conducted with a maximum blowing ratio of 4.7 and a dimensionless oscillation frequency of 0.65. The outward pulse from the jets in each oscillation cycle causes a disturbance to move down the airfoil surface. The leading and trailing edge celerities for the disturbance match those expected for a turbulent spot. The disturbance is followed by a calmed region. Following the calmed region, the boundary layer does separate, but the separation bubble remains very thin. Results are compared to an uncontrolled baseline case in which the boundary layer separated and did not reattach, and a case controlled passively with a rectangular bar on the suction surface. The comparison indicates that losses will be substantially lower with the jets than in the baseline or passively controlled cases.

Dynamic Stall Control Using a Model-Based Observer

Dynamic stall control is of interest for its potential to enhance the performance of rotorcraft. The experiments described used pulsed vortex generator jets, shown previously to prevent separation on steady wings and airfoils, to delay dynamic stall on a rapidly pitching airfoil. In tests performed on a NACA-0012 airfoil at low Reynolds numbers, the jets yielded a 25‐ 35% increase in the lift coefe cient achievable without moment stall. Next, a differential equation model of the dynamic stall process was derived and experimentally validated. Models of this form enable the application of well-known control theory to the design of e ow control systems. In the e nal round of experiments, a state observer based on the mathematical model detected incipient dynamic stall. A controller used the stall prediction to operate the pulsed jets only when needed to prevent stall. The controller turned the jets off for the remainder of the oscillation cycle. The controller only operated the jets for about 25% of the pitch cycle, so that the best lift increases of the early experiments could be achieved with only 25% of the air mass e ow.