Steady and Unsteady Excitation of Separated Flow over the NASA Hump Model

Abstract

Separated flow and its control over the NASA hump model were investigated at subsonic speeds. Three-dimensional, unsteady fluid dynamic simulations were supplemented by wind tunnel measurements. Flow control was implemented by means of spatially distributed discrete jets operating in steady and unsteady modes. The effects of excitation amplitude and frequency were studied numerically and experimentally. Several integral parameters were explored as quality metrics. In addition to the existing ones, two new integral parameters, which are slightly modified versions of the normal force and moment coefficients, were introduced. These two parameters together with pressure drag coefficient were found to be well correlated with the flow control and used in the performance evaluation of different flow control methods including, zero net mass flux actuators, steady suction, sweeping jet actuators, and the currently studied steady and unsteady excitations. For the cases tested, it was found that the unsteady excitation is superior to the steady excitation and slightly better than the sweeping jet actuators whereas the steady suction was found to be the most effective. Although the numerical simulations overpredict the separation bubble, these simulations capture the salient features of flow separation control and hence help us to understand the effect of steady/unsteady excitation on the separated flow. Nomenclature c = hump chord Cdp = pressure drag coefficient Cinv = inviscid flow comparison coefficient Cmq = moment coefficient based on quarter-chord Cmx = moment coefficient based on xm Cµ = jet momentum coefficient Cn = normal force coefficient Cnp = modified normal force coefficient Cp = pressure coefficient Rec = Reynolds number based on hump chord x, y, z = streamwise, vertical, and spanwise directions xm = hump slope discontinuity xr = reattachment location xs = separation location

Performance Assessment of Fluidic Oscillators Tested on the NASA Hump Model

“…Unlike the unsteady synchronized actuations in Refs. [15,21], the random oscillations of the fluidic oscillator jet do not cause vortex shedding associated with the flow separation bubble. Fluctuating pressures in the pressure recovery region with varying momentum coefficient are presented in Figure 10b.…”

“…The pressure appears to be recovered downstream of the model trailing edge near x/c = 1.3 due to the separated flow and the splitter plate. The inviscid flow solution of the three-dimensional hump model [21] is also presented in this figure. The ideal (i.e., inviscid) flow over the hump model results in more flow deceleration near the leading edge of the model and more acceleration afterward.…”

“…Rather, the periodic fluctuations shown in the closest pressure ports are local periodic fluctuations due to the fluidic oscillator flow field. On the other hand, the unsteady actuations using two-dimensional ZNMF actuators (with the same drive signal) or discrete synchronized pulsed jets were shown to cause the generation, rollup, and shedding of the vortex associated with flow separation bubble, which resulted in coherent surface pressure fluctuations [15,21]. One of the reasons for not seeing a clear vortex shedding is possibly due to the random oscillation of each fluidic oscillator in the actuator array.…”

Application of synthetic jet arrays for flow separation control on a circular “hump” model

“…Results show that fluidic oscillators are almost always superior to steady jets with respect to any flow control coefficient at a given spacing. Koklu (2018), studied the effects of excitation amplitude and frequency, both numerically and experimentally, on separated flow and its control over the NASA hump model at subsonic speeds by using discrete jets operating in steady and unsteady modes. It was revealed that the unsteady excitation is superior to the steady excitation and a bit better than the sweeping jet actuators while the steady suction was found to be the most effective.…”