Whitham modulation theory for generalized Whitham equations and a general criterion for modulational instability. Issue 2 (25th May 2021)

• Record Type: Journal Article
• Authors: Binswanger, Adam L.; Hoefer, Mark A.; Ilan, Boaz; Sprenger, Patrick
• Is Part Of: Studies in applied mathematics. Volume 147:Issue 2(2021)
• Page Start: 724
• Page End: 751
• Publication Date: 2021-05-25
• Subjects: modulational instability -- the Whitham equation -- Whitham modulation theory; Mathematics -- Periodicals; Mathématiques -- Périodiques; Mathématique; Mathématique appliquée; Ressource Internet (Descripteur de forme); Périodique électronique (Descripteur de forme); 519
• DOI: 10.1111/sapm.12398
• Languages: English
• ISSNs: 0022-2526

Abstract

Abstract: The Whitham equation was proposed as a model for surface water waves that combines the quadratic flux nonlinearity f ( u ) = 1 2 u 2 of the Korteweg–de Vries equation and the full linear dispersion relation Ω ( k ) = k tanh k of unidirectional gravity water waves in suitably scaled variables. This paper proposes and analyzes a generalization of Whitham's model to unidirectional nonlinear wave equations consisting of a general nonlinear flux function f ( u ) and a general linear dispersion relation Ω ( k ) . Assuming the existence of periodic traveling wave solutions to this generalized Whitham equation, their slow modulations are studied in the context of Whitham modulation theory. A multiple scales calculation yields the modulation equations, a system of three conservation laws that describe the slow evolution of the periodic traveling wave's wavenumber, amplitude, and mean. In the weakly nonlinear limit, explicit, simple criteria in terms of general f ( u ) and Ω ( k ) establishing the strict hyperbolicity and genuine nonlinearity of the modulation equations are determined. This result is interpreted as a generalized Lighthill–Whitham criterion for modulational instability.