T. T. Ng

Bio: T. T. Ng is an academic researcher from University of Notre Dame. The author has contributed to research in topics: Vortex & Delta wing. The author has an hindex of 5, co-authored 8 publications receiving 258 citations.

Visualization and wake surveys of vortical flow over a delta wing

Abstract: An experimental investigation of vortex breakdown on delta wings at high angles of attack is presented. Smoke flow visualization and the laser light sheet technique were used to obtain cross-sectional views of the leading-edge vortices as they break down for a series of flat-plate delta wings having sweep angles of 70, 75, 80, and 85 deg. At low tunnel speeds (as low as 3 m/s), details of the flow that are usually imperceptible or blurred at higher speeds can be clearly seen. A combination of lateral and longitudinal cross-sectional views provides information on the three-dimensional nature of the vortex structure before, during, and after breakdown. Whereas details of the flow are identified in still photographs, the dynamic characteristics of the breakdown process have been recorded using high-speed movies. Velocity measurements have been obtained using a laser Doppler anemometer with the 70 deg delta wing at 30 deg angle of attack. The measurements show that, when breakdown occurs, the core flow is transformed from a jet-like to a wake-like flow.

Visualization and flow surveys of the leading edge vortex structure on delta wing planforms

Abstract: In the present experimental investigation of thin delta wing vortex breakdown, for the cases of sweep angles of 70, 75, 80, and 85 deg, and smoke flow visualization/laser light sheet technique is used to obtain cross sectional views of the leading edge vortices as they break down. A combination of lateral and longitudinal cross sectional views furnishes data on the three-dimensional character of the vortex before, during, and after breakdown. Velocity measurements conducted with a laser Doppler anemometer on the 70 deg sweep delta, at 30 deg angle-of-attack, indicate that when breakdown occurs the core flow is transformed from a jet-like to a wake-like flow.

Experimental study of the velocity field on a delta wing

Abstract: An experimental study of the leading edge vortices on delta wings at large angles of incidence is presented. A combination of flow visualization, seven-hole pressure probe surveys and laser velocimeter measurements were used to study the leading edge vortex formation and breakdown for a set of delta wings. The delta wing models were thin flat plates with sharp leading edges having sweep angles of 70, 75, 80, and 85 degrees. The flow structure was examined for angles of incidence from 10 to 40 degrees and chord Reynolds numbers from 85,000 to 640,000. Vortex breakdown was observed on all the wings tested. Both bubble and spiral modes of breakdown were observed. The visualization and wake survey data shows that when vortex breakdown occurs the core flow transforms abruptly from a jet-like flow to a wake-like flow. The result also revealed that probe induced vortex breakdown was more steady than the natural breakdown.

Seven hole probe measurement of leading edge vortex flows

Abstract: This paper discusses the use of a seven-hole probe on measurements of leading edge vortices of highly sweep delta wing planforms. Intrusive probe data taken with the pressure probe were compared with nonintrusive measurements made with laser Doppler anemometry system. In addition to probe size, the natural position of breakdown and the sweep angle of the wing are also factors in determining sensitivity of the flow to probe interference. At low angles of attach vortex breakdown does not occur in the vicinity of the model and the seven hole probe was found to yield reasonably accurate measurements. When the angle of attack of the model was increased so that vortex breakdown was near the trailing edge, introducing the probe over the wing would cause the breakdown position to move ahead of the probe. However, when breakdown naturally occurred ahead of the mid-chord of the wing the vortices were found to be less sensitive to a probe placed behind the breakdown point. Vortex breakdown on a lower swept wing is found to be more sensitive to interference. Near the breakdown region, seven hole probe measurement is less accurate due to a combination of probe interference and flow reversal.

Phase-locking in a jet forced with two frequencies

Abstract: This study investigates the effect of forcing a shear layer at more than one frequency. Multiple frequency forcing permits the phase and initial relative amplitudes among unstable waves to be manipulated. More control can be imposed on vortex merging and mixing. Various vortex merging modes were observed and explained by the relative strength of the instability waves and their phase alignment. The vortex phase and path jitterings present in single-frequency forcing cases are greatly reduced when forced at more than one frequency. The observed cycle-to-cycle variation was small. This enables phase-lock measurements to be performed more easily. The phase-lock data show excellent agreement with the flow visualization results even when averaged over only a few cycles.

Navier-Stokes Computations of Vortical Flows Over Low-Aspect-Ratio Wings

Abstract: An upwind-biased finite-volume algorithm is applied to the low-speed flow over a low aspect ratio delta wing from zero to forty degrees angle of attack. The differencing is second-order accurate spatially, and a multigrid algorithm is used to promote convergence to the steady state. The results compare well with the detailed experiments of Hummel (1983) and others for a Re(L) of 0.95 x 10 to the 6th. The predicted maximum lift coefficient of 1.10 at thirty-five degrees angle of attack agrees closely with the measured maximum lift of 1.06 at thirty-three degrees. At forty degrees angle of attack, a bubble type of vortex breakdown is evident in the computations, extending from 0.6 of the root chord to just downstream of the trailing edge.

Review of Unsteady Vortex Flows over Slender Delta Wings

Abstract: Nomenclature AR = aspect ratio; amplitude ratio B = probability density function of velocity c = root chord length f = frequency k = reduced frequency; axial wave number n =w ave number in angular direction P = probability p = pressure fluctuation Re =R eynolds number based on chord length S = spectral density s = local semispan T = period t = time U∞ = freestream velocity u = axial velocity v = swirl velocity x = streamwise distance xbd = breakdown location y = spanwise distance z =v ertical distance above wing surface α = angle of attack � = circulation δ = flap angle � = sweep angle ν = kinematic viscosity τ = time constant � =fi nangle ω =v orticity; radial frequency