Berl Oster

... 30 1.50 800 40 0.75 600 60 0.40 400 Fig. 2. Velocity - and turbulence intensity distribution. - flap inoperative - U c= 10 mlsec (Uc=(UI + U2)12 , Oster [ roughly a constant: St = fxs / U c = 2.4 Figs. 3a, b, c suggest that at large x (x all forced oscil]ation cases (A > 0.3) to ...

Measurements were carried out in an isothermal plane mixing-layer in absence of pressure gradient and under controlled initial conditions. Disturbances of known frequency and amplitude were introduced into the flow at its origin while measurements were conducted further downstream. Constant temperature hot-wire anemometers were used to measure the mean velocity, the shear stress and the three components of turbulent intensity for various initial conditions. The flow is not self preserving in the range of Reynolds numbers considered. The initial spreading rate of the mixing layer depends on the amplitude and the frequency of the imposed oscillations. At a given frequency, the rate of spreading increases initially with the amplitude of the imposed oscillation but a saturation region occurs beyond which one can no longer control the flow. The spreading rate in the saturation region is less than for the undisturbed mixing layer. The initial rate of spread of the mixing layer is associat...

The effect of periodic two-dimensional excitation on the development of a turbulent mixing region was studied experimentally. Controlled oscillations of variable ampli- tude and frequency were applied at the initiation of mixing between two parallel air streams. The frequency of forcing was at least an order of magnitude lower than the initial instability frequency of the flow in order to test its effect far downstream. The effect of the velocity difference between the streams was also investigated in this experiment. A typical Reynolds number based on the velocity difference and the momentum thickness of the shear layer was l04.It was determined that the spreading rate of the mixing layer is sensitive to periodic surging even if the latter is so small that it does not contribute to the initial energy of the fluctuations. Oscillations at very small amplitudes tend to increase the spreading rate of the flow by enhancing the amalgamation of neighbouring eddies, but at higher amplitude...

Barrier island vulnerability to storm-generated waves is directly related to interactions between shoreface morphology and surf-zone dynamics. During storms, the seaward-most dune often limits the landward extent of wave energy; however, if maximum wave run-up exceeds the elevation of the top of the dune, overwash or inundation may occur. The `Storm Impact Scale' presented by Sallenger (2000) classifies barrier beach vulnerability to individual storm events based on the elevation of the frontal dune crest and toe relative to maximum wave run-up. Changes to the dune and beachface can occur over a range of time scales, altering local vulnerability to extreme waves from storms, even as a storm is occurring. As sea level continues to rise, barrier beaches will become increasingly vulnerable to overwash and inundation from a greater number of storms. Our objective is to assess temporal trends in barrier island vulnerability while also exploring island-chain-wide response and recover...

The effect of a trip wire on the large structure in the mixing layer is examined at moderately high Reynolds numbers. It is observed that the trip wire may either enhance or inhibit the spreading rate of the mixing layer depending on the velocity ratio between the two streams. The effect of the trip wire extends downstream beyond the 1000 initial momentum thicknesses and is felt in a region which is supposed to be self-preserving. Correlation and spectral results indicate that the large structures in the mixing layer consist of quasi two-dimensional row of vortices which are convected approximately at the average velocity of the two streams. The abrupt change in the predominant wave length of the correlations with downstream distance suggest that vortices interact and possibly pair while being convected.