Richard W. Miksad

Measurements of nonlinear interactions during natural transition of a symmetric wake

Bicoherency spectrum measurements are used to study the nonlinear dynamics of spectral broadening during natural transition of a two‐dimensional wake. Dominant nonlinear interactions are identified. The results are compared with multiple instability controlled transition experiments.

Fundamental-subharmonic interaction : effect of phase relation

The effect of the phase relation (i.e. phase difference and coupling) between the fundamental and subharmonic modes on the transition to turbulence of a mixing layer is investigated. Experiments are conducted to study the development of the subharmonic and fundamental modes under different phase-controlled excitations. Higher-order spectral moments are used to measure phase differences, levels of phase coupling, and energy transfer rates between the two modes at different downstream locations. Local measurements of the wavenumber–frequency spectra are used to examine the phase-speed matching conditions required for efficient energy transfer. The results show that when the phase coupling between the fundamental and the subharmonic is high, maximum subharmonic growth is found to occur at a critical phase difference close to zero. The subharmonic growth is found to result from a resonant parametric interaction between the fundamental and the subharmonic in which phase-speed matching conditions are satisfied. In contrast, when the phase coupling level is low, the phase difference is irregular and varying, the efficiency of parametric interactions is low, phase-speed matching conditions are not met and subharmonic growth is suppressed.

Nonlinear spectral dynamics of a transitioning flow

The nonlinear spectral dynamics of a transitioning flow in the wake of a flat plate is experimentally studied at different downstream positions with a two‐point method. The measurement setup consists of two sensors, which are separated in the downstream direction. The quadratically nonlinear transfer function between the two points is then estimated from the digitized fluctuation data. Such transfer functions permit one to quantify the quadratically nonlinear spectral dynamics occurring between the two sensor points. The method used to estimate the transfer functions and local bicoherency for non‐Gaussian input and output signals, by means of digital spectral analysis techniques, is briefly discussed. The measured quadratic transfer function of the experimental data changes gradually with downstream distance, but its main features are unchanged. The observed appearance of progressively higher harmonics of the fundamental mode and the filling in of the spectral valleys over short downstream distances are, ...

Application of digital cross-bispectral analysis techniques to model the non-linear response of a moored vessel system in random seas

Abstract Digitally implemented cross-bispectral analysis techniques are utilized to model the quadratically non-linear response of moored vessels subject to random seas. Specifically, the model, which is based on a Volterra series representation, consists of a parallel combination of linear and quadratic transfer functions. The key ideas underlying the estimation of the quadratic transfer function by applying digital cross bispectral analysis to the sea wave input and moored vessel response data are described. The validity and usefulness of the approach are demonstrated with actual model basin test data for the non-linear frequency response of a moored vessel subject to random seas.

Measurement of the local wavenumber and frequency spectrum in a plane wake

A digital technique is presented for experimentally measuring the local wavenumber-frequency spectrum SL(k, f) of a fluctuating velocity field using two probes. From SL(k, f), the local wavenumber spectrum SL(k), the averaged dispersion relation, and the broadening of the wavenumber spectrum for a given frequency can be determined. The technique is demonstrated by applying it to the velocity field of a plane wake which is undergoing transition from laminar to turbulent flow. A specially designed two sensor hotwire probe is used to obtain simultaneous records of streamwise velocity fluctuations at two locations having a fixed streamwise separation. New information is obtained concerning: the spatial characteristics of instability waves in the wake; the importance of local wavenumber matching in nonlinear coupling among waves; and broadening of the dispersion relation associated with the transition to turbulence. The usefulness of the SL(k, f) approach when Taylors hypothesis is not valid is also discussed.

A Simple Method for Estimating the Influence of Cloud Cover on the NO2 Photolysis Rate Constant

A method based on hourly NWS cloud amount reports is presented for developing a simple model to account for cloud cover in the determination of the nitrogen dioxide photolysis rate constant, k 1 The model is parameterized and verified with direct UV radiometer and k 1 measurements (vs. time of day) collected by Sickles, et al. 1 at Research Triangle Park. Application of our model to variable cloud condition situations indicates that significant improvement in k 1 prediction is obtained by including the influence of cloud cover. Comparison of our model with the radiative transfer calculations of Peterson7 indicates that the particular parameterization of k 1 given here is most representative of average albedo and relatively heavy aerosol loading conditions. Comparison of ozone predictions using hourly averaged k 1 and instantaneous k 1 under conditions of varying cloud cover suggest that the errors resulting from averaging k 1 are largest when variations in solar zenith angle are significant over the hour.

A Simple Model for Urban Ozone Impact Predictions

A simple urban ozone model for air quality management analysis is presented. The model is evaluated by comparing predicted and observed patterns in monthly average ozone maxima and dosages for two distinct urban areas and by comparing the simple model results for altered sources to the results of the Empirical Kinetic Modeling Approach (EKMA) and the Graedel complete urban air chemistry model. From these comparisons it is concluded that the simple ozone model can be used for accurate yet efficient qualitative analysis of the influence of various air quality policy options on future ozone impacts.

A technique to measure wavenumber mismatch between quadratically interacting modes

Nonlinear energy cascade by means of three-wave resonant interactions is a characteristic feature of transitioning and turbulent flows. Resonant wavenumber mismatch between these interacting modes can arise from the dispersive characteristics of the interacting waves and from spectral broadening due to random effects. In this paper, a general technique is presented to estimate the average level of instantaneous wavenumber mismatch, 〈Δk〉=〈km-ki-kj〉, between components whose frequencies obey the resonant selection condition, fm-fi-fj=o. Cross-correlation of the auto-bispectrum is used to quantify the level of mismatch. The concept of bispectrum coupling coherency is introduced to determine the confidence level in the wavenumber mismatch estimates. These techniques are then applied to measure wavenumber mismatch in the transitioning field of a plane wake. The results show that the average of the instantaneous mismatch between the actual interacting modes 〈km-ki-kj〉 is in general not equal to the mismatch between the average wavenumbers of each interacting mode 〈km〉-〈ki〉-〈kj〉.

Measurement of Nonlinear Transfer Functions for Transitioning Flows

A method for estimating the linear and quadratically nonlinear transfer functions between two points in a transitioning or turbulent flow is presented. The technique does not require a Gaussian input signal, and is demonstrated on data measured in the nonlinear stages of the transitioning flow of a wake behind a thin flat plate. The results indicate that in the early nonlinear stages of transition, interactions between modes of similar frequency and a low frequency component are most efficient in redistributing spectral energy. As transition progresses, interactions involving the first harmonic frequency become efficient as well in the spectral energy transfer process. In general, nonlinear transfer function techniques allow a quantitative characterization of spectral energy transfer dynamics.

Experiments on the Natural Transition of an Unsteady Wake

Experiments were conducted to investigate the natural transition to turbulence of a periodically unsteady laminar wake. Measurements show that periodic mean flow unsteadiness acts to modulate instability fluctuations in amplitude and phase and as a result increases spectral broadening. Significant nonlinear wave coupling between the unsteady mean flow and the generated sidebands is found in bicoherence spectra calculations. The results of these experiments, and other controlled transition experiments where specific instabilities are excited, indicate that an unsteady mean flow can act as a catalyst to accelerate the transition by enhancing the generation of sideband energy which can ultimately be redistributed through nonlinear interactions.