# Richard Walter MiksadUniversity of Virginia | UVa · School of Engineering and Applied Science

Richard Walter Miksad

Sc.D.

## About

49

Publications

552

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674

Citations

## Publications

Publications (49)

Knowledge of the frequencies and amplitudes and single-mode versus multi-mode behavior of deepwater riser responses is important for accurate fatigue analysis. The extraction of this information from measured riser responses is a significant challenge given the complexity of the phenomena involved with riser vibrations. An existing technique for ob...

In this work, wavelet analysis is applied to measured responses of a 1:55 scale-model spar platform in order to measure instantaneous amplitude and frequency fluctuations of surge and pitch natural modes. The analysis was performed for a spar subjected to irregular wave loading representative of operational conditions, a 10-year storm, and a 100-ye...

Wavelet analysis is used to determine time-varying characteristics of unstable modes in the transistion of plane mixing layers. Steady and unsteady mean flow cases are considered. The results show that, beyond the region of linear stability, the growth of the fundamental and subharmonic is synchronized in time. This synchronization is evidence of t...

Wavelet transform of u' velocity fluctuation at successive streamwise locations in a transitioning wake reveal that a large extent of transition can be characterized as a combination of sinusoidal fluctuations from the Rayleigh instability and intermittent time-localized events which appear as vertical cone-like features in visualizations of wavele...

Wavelet analysis is used to determine time-varying characteristics of unstable modes in the transistion of plane mixing layers. Steady and unsteady mean flow cases are considered. The results show that beyond the region of linear stability, the growth of the fundamental and subharmonic is synchronized in time. This synchronization is evidence of th...

Spatial coherence of the streamwise velocity fluctuation was measured in the wake of a flat plate in a wind tunnel. The measurements were made in the context of determining the role of wavenumber resonance in quadratic interactions among resonant frequency modes. Frequency domain results show that sum-interaction modes targeted by coupled mode pair...

The wavelet transform has emerged as an important tool for the analysis of intermittent and nonstationary signals. This article addresses implementation issues of a digital continuous wavelet transform that is sufficiently general that any continuous wavelet can be used. Working equations are derived for wavelet sampling, aliasing, and scale popula...

In fluid, flows, fluid-structure interactions and other fluids-related problem, nonlinear dynamics play an important role in determining the development, response or output. Understanding these dynamics is essential for development of analytical models and prediction and control purposes. Higher-order statistical analysis has been shown to be an ef...

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 techn...

This paper presents an adaptive second-order Volterra filter and
its application to model-test data of a prototype tension leg platform
(TLP). The least-squares approach of a second-order Volterra model and
its adaptive filtering algorithm based on recursive least-squares are
introduced. The second-order Volterra filter is applied to identify the
l...

An experimental investigation is conducted to determine the effect of low-frequency mean flow unsteadiness on the characteristics of a plane wake. Complex demodulation and bispectral analysis techniques are used to obtain quantitative measurements of the modulation and coupling mechanism generated by the mean flow unsteadiness. The results show tha...

The phase and magnitude of the auto-bispectrum are used to locally measure the phase difference and coupling level between the fundamental and subharmonic components of a free shear layer. Two cases in which both modes are excited at different relative phases are examined. The results show that the subharmonic growth is continuous beyond the linear...

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...

A technique is presented to examine the level of wavenumber mismatch between nonlinearly interacting components whose frequencies obey resonant matching conditions. Cross-correlation of the auto-bispectrum is used to quantify the level of mismatch, and the concept of bispectrum coherency is introduced to determine the confidence level in the wavenu...

Higher-order spectral analyses of the velocity fluctuations in a transitioning mixing layer are presented. The phase of the auto-bispectrum is used to measure the phase difference between the fundamental and subharmonic modes. The normalized magnitude of the auto-bispectrum is used as an indication of the goodness of the measured phase difference....

The generation of low-frequency velocity components and their influence on the dynamics of a plane wake are examined. The results indicate that the influence and role of low-frequency modes on wake transition and spectral energy transfer depends somewhat on the mechanism by which these modes are generated. In both steady and unsteady wakes, low fre...

An experimental investigation is conducted in order to quantify the nonlinear and parametric resonance mechanisms that are associated with the subharmonic growth in the transition to turbulence in plane mixing layers. Higher-order digital statistical analysis techniques are used to investigate the nonlinear and parametric mechanisms responsible for...

Modeling and forecasting of the sway of a moored vessel subjected to random waves were studied recently using quadratic digital filtering techniques. Moreover, it was observed that the future response of the vessel can be predicted with relatively high accuracy. This paper describes a control scheme, which utilizes a quadratic digital filter, to st...

The effect of forcing the fundamental and subharmonic modes at different initial phases on the subharmonic growth in a transitioning free shear layer is investigated. The variations in the development of the subharmonic and fundamental modes under natural and phase controlled excitation are pointed out. Statistical measurements and analysis are use...

The effect of introducing a low-frequency periodic mean flow component on the subharmonic growth is investigated. The differences in the development of the fundamental and subharmonic modes under steady and unsteady mean flow conditions are pointed out. The causes behind these differences are investigated using higher order digital analysis techniq...

An experimental investigation was conducted to quantify the nonlinear mechanisms that are responsible for the growth of the subharmonic mode in the transition to turbulence in a plane mixing layer. Polyspectral measurements and analyses were used to provide experimental proof of the parametric resonance between the fundamental and the subharmonic t...

The nonlinear mechanisms responsible for the generation of subharmonic fluctuations on a transitioning mixing layer are determined by using polyspectral transfer function techniques. Cross-bispectral analysis is used to identify three-wave couplings, and quadratic transfer functions are measured to determine the spectral transfer of energy due to n...

By modelling the sway response of a tension leg platform (TLP) in terms of a second-order Volterra system, we are able to decompose both spectrally and temporally the response into its linear and quadratically nonlinear components. The approach is demonstrated with the aid of experimental data of TLP sway motion obtained from a scaled (1:54) model...

In this paper we consider two second order Volterra filters that are useful in modeling low-frequency drift oscillation phenomena. In the first case, the earlier work of Koh et al. is generalized to include excitation by non-Gaussian irregular seas. In the second case, this filter is extended to an effective infinite impulse response second order V...

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 f...

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 estimate...

A digital technique is presented for experimentally measuring the local wavenumber-frequency spectrum S
L(k, f) of a fluctuating velocity field using two probes. From S
L(k, f), the local wavenumber spectrum S
L(k), the averaged dispersion relation, and the broadening of the wavenumber spectrum for a given frequency can be determined. The technique...

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
indicat...

A quadratic system model based on Volterra series representation is utilized to model the nonlinear response of moored vessels subjected to random seas. The key idea is to represent the relationship between the incident sea wave (input) and corresponding sway response of the moored vessel (output) with a parallel combination of linear and quadratic...

The use of digital analysis techniques to extract information on
nonlinear wave interactions from experimental data is described and
demonstrated, summarizing the results of recent investigations. The
concept of polyspectra is introduced; digital techniques for bispectra
are outlined; and the detection of three-wave interactions in raw
fluctuation...

The transition to turbulence in the wake flow behind a flat plate is investigated experimentally at freestream velocity 8 m/s, Reynolds number 550, and background turbulence level 0.05 percent in the subsonic wind-tunnel facility described by Miksad et al. (1982). Results for both natural transitions (induced by the random acoustic background) and...

This paper is concerned with overviewing unique state-of-the-art digital
signal processing techniques that are capable of yielding new
experimental insight into various nonlinear mechanisms associated with
transition to turbulence. The paper focuses on a technique whereby the
omega,k (temporal frequency-wavenumber) power spectrum can be estimated
f...

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 esti...

Low-frequency difference modes are generated in the earliest stages of transition through nonlinear difference interactions between growing instabilities. The generation of difference modes, and their influence on the subsequent stages of transition is sensitive to the relative amplitudes and frequencies of the interacting instabilities. Phase (fre...

The problem of modelling and forecasting the sway motion response of a moored vessel to random sea waves is discussed. When the sea wave fluctuation is measured and recorded digitally with a proper sampling rate, the future response of the vessel can be actually predicted with relatively high accuracy by utilizing an (adaptive) quadratic filtering...

Bispectral analysis measurements are used in conjunction with power spectra and probability density function measurements to study the quantitative features of the nonliner interactions which govern the energy redistribution and randomization processes during natural transition of a two-dimensional wake. Dominant nonlinear interactions are identifi...

The generation of new fluctuations by nonlinear interactions during the transition of a two-dimensional wake is studied. Spectral energy transfers are estimated by using digital bispectral analysis techniques to determine coupling coefficients and power transfer functions.

The calculation of wave drift forces by using digital complex demodulation to recover the amplitude envelope of an irregular wave train is discussed. Results of scaled (1:48) model basin tests of a moored barge in an irregular sea are used to demonstrate the practicality of the technique. It is also shown that the spectral density of the sway respo...

The transition of a laminar two-dimensional wake is studied experimentally to establish the role of amplitude and phase modulations in the spectral-broadening and energy-redistribution process. Multiple instability modes fo and fi are triggered by acoustic excitation. The spectrum of the fluctuating velocity field formed by the growing and interact...

The authors discuss digital bispectral analysis, which is a very useful tool for the study of nonlinear wave coupling in fluids. Furthermore, the coupling coefficient is derived in terms of the bispectrum and is estimated for the sum of difference interactions in a 2D shear flow. It is found that the difference interaction is stronger than the sum...

The relationship between nonlinear wave coupling and the properties of the bispectrum are investigated for the case of three-wave coupling. In particular, the dependence of the phase of the bispectrum, the direction of power flow between modes, and the sign of the skewness parameter, on the nonlinear amplitude variation and coupling coefficient, is...

The continuous wavelet transform is applied to examine u' velocity fluctuations measured in an unseparated wake undergoing transition to turbulence. The objective of this work is to investigate the development of large scale intermittent fluctuations which are detected in the velocity fluctuations as spike-like events. Observation of the Morlet wav...

Partial reconstruction of wavelet transforms is a technique by which the time-dependent amplitude envelopes of individual modes can be extracted from a response measurement. It has been shown to be an accurate technique for extracting amplitude envelopes when the modes are well separated in wavelet space. In this work, the ability of partial recons...

It is the purpose of this paper to describe a new approach that has recently been developed for analyzing, modeling, and interpreting nonlinear systems, when the nonlinear system is excited by non-Gaussian inputs. The technique should have wide application in many areas of offshore engineering. In this paper, the validity of the approach has been d...