Tadashi Morioka

Dynamic Characteristics in a Control System of Backward Facing Step Flow by Vortex Generator Jets

*† The present study is conducted in an attempt to construct a closed loop control system of the reattachment process for a backward facing step flow by using a row of vortex generator jets. Two-dimensional reattachment is found to occur when the spanwise pitch of the injected jets is 10d to 20d, where d is the jet diameter, because measurement of the forward flow fraction indicates that the reattachment line is approximately parallel to the spanwise direction. It is necessary to examine the dynamic characteristics of the injection jet on the reattachment process in the design of the control system. The coherence and the frequency transfer characteristics between the injection jet and the near-wall flow in the reattachment region are examined in an open-loop system. The experimental result shows that the coherence of the jet velocity with the surface static pressure is higher than that with the surface friction velocity. The surface static pressure has better frequency transfer gain characteristics near the reattachment point. A step response of the near-wall flow is also conducted in order to investigate the transient characteristics. Damped oscillation with respect to the surface static pressure is found to occur immediately after a step change in the jet velocity.

Flow Behavior in a Dump Diffuser With Distorted Flow at the Inlet

The cold flow experiments are made in a two-dimensional dump diffuser. Behavior of an impinging jet over a flame dome head and its reattachment to the casing wall is presented, when a symmetric or distorted flow is provided at the diffuser inlet. A hot-wire anemometer and laser doppler velocimeter are used. Diffuser performances of the pressure recovery and total pressure loss on three types of pre-diffuser geometry and four cases of dump gap between pre-diffuser exit and flame dome head are tested.Both pre-diffuser geometry and dump gap are dominant parameters affecting pressure recovery and total pressure loss. A small dump gap causes insufficient pressure recovery in the pre-diffuser, resulting in a loss of total pressure in the diffuser. Distribution of the flow rate to the branched channels is relatively insensitive to the inlet velocity profile with distortion, as the shift of the jet stream occurs just upstream of the flame dome head.Copyright

A Role of Longitudinal Vortex on the Separating Shear Layer Development over a Backward Facing Step

υu − Reynolds shear stress uw − Reynolds shear stress u* Friction velocity Vj Velocity of injected jet VR Ratio of jet velocity to free stream velocity X, Y, Z Streamwise, transverse and spanwise coordinate y + Transverse distance (=Y / ) Zi Auxiliary spanwise coordinate (origin at midpoint between injection holes at step span center) ν Kinematic viscosity ρ Density

The Response of Injection of Vortex Generator Jets to a Backward Facing Step Flow

In this paper, the flow behavior of a reattachment process over a backward facing step flow is reported. The reattachment process is controlled by injection of vortex generator jets. The injection of jets upstream of the step produces the co-rotating longitudinal vortices in a separating shear layer. The experiment of the step response of the injection jet is also conducted in order to investigate the evolution process of the longitudinal vortices. A large scale of primary and counter vortices are observed, when the velocity ratio of the free stream to injected jet is 6. The detailed structure of the longitudinal vortices is clarified. The remarkable effect of the vortices on the separating shear layer downstream of the step is observed.Copyright

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.